The findings of the survey of 1,000 parents, conducted online by the research charity Autistica, has exposed the lack of help for those affected.

The chief complaint among those surveyed was the length of time it took to reach a diagnosis.

Despite there being no proven treatment for the condition, parents wanted to know as early as possible what was wrong with their child.

Olivia Curno, spokesperson for the charity, which has raised £4m for autism research since it was founded in 2004, said: "Parents see their friends' children developing and wonder why their own do not respond in the same way. They ask themselves: 'What am I doing wrong?' They are struggling for a diagnosis at a difficult time."

It can take three years before a diagnosis is reached.

Ms Curno said differences in responses to social cues were evident in autistic children from the first few months and neurological signs could be seen at eight months.

But they were not yet sufficiently well established to allow a diagnosis to be made.

"At the moment autism is diagnosed on the basis of social factors, so it is somewhat subjective.

"These observations are the first steps toward establishing earlier diagnosis with a biological test, which would be quicker and more impartial."

The survey found half of parents who saw their GP about their child's autism had a "negative" experience.

Though some practice was good, it was widely variable.

Three-quarters said they had received no specific help but would have liked support.